1. Field of the Invention
This invention relates to a method for the production of a porous material attained by the formation of a water-in-oil type high internal phase emulsion and characterized by causing the waste water used in forming the water phase in the formation of the water-in-oil type high internal phase emulsion to be reused at any of the steps of process of the production.
2. Description of Related Art
For the production of a porous material consisting of uniform open cells of a minute diameter, a method which make a polymer form in a water-in-oil type high internal phase emulsion in the presence of a specific surfactant is available. The term "high internal phase emulsion" as used herein is generally construed as referring to an emulsion such in constitution that the ratio of an internal phase to the whole volume of the emulsion exceeds 70 vol. % (K. J. Lissant, Journal of Colloid and Interface Science, Vol. 22, p. 462 (1966)). U.S. Pat. No. 5,334,621, for example, discloses a method for producing a porous material in accordance with the water-in-oil type high internal phase emulsion method in which polymerizable monomers contained in such a water-in-oil type high internal phase emulsion are cross-link-polymerized in the emulsion (hereinafter referred to simply as HIPE method).
The HIPE method produces a porous material by preparing a water-in-oil type emulsion comprising (i) a polymerizable monomer mixture containing an oil-soluble vinyl monomer and cross-linking monomer having not less than two functional groups in the molecule, (ii) a water phase of such a quantity as to occupy 90 wt. %, preferably 95 wt. %, and particularly preferably 97 wt. %, of the emulsion, (iii) a surfactant such as a sorbitan fatty acid ester and a glycerol monofatty acid ester, and (iv) a polymerization initiator and heating the water-in-oil type emulsion thereby inducing polymerization and cross-linkage thereof. By this HIPE method, a porous material containing open cells after the fashion of a network is formed in consequence of reversed-phase emulsion polymerization. The porous material to be obtained by the HIPE method, therefore, acquires low density and such characteristic properties as ability to absorb water, ability to retain absorbed water, ability to provide heat insulation, and ability to abate noise.
A porous material of low density, namely high porosity, proves to be useful for applications enumerated hereinbelow and nevertheless manifests inferior efficiency in terms of transportation and storage because of high bulkiness. For the purpose of eliminating this drawback, International Publication No. 96/40,823, for example, discloses a method for producing a compressed porous sheet having a thickness of one in several parts of the heretofore usual thickness by polymerizing a porous cross-linked polymer in accordance with the HIPE method. This compressed porous sheet has the nature of retaining the compressed state and, on exposure to a large amount of liquid, quickly absorbing the liquid and restoring the original thickness.
The porous material according to the HIPE method utilized in this manner, however, leaves the formation of pores therein to be affected by the ratio of the water phase as an internal phase to the oil phase as an external phase, i.e. W/O, during the reversed-phase emulsion polymerization in the process of production thereof. An effort to obtain this porous material with as large a pore volume ratio as permissible inevitably results in heightening the proportion of the water phase side of the ratio of W/O. The porous materials of this class enjoy a huge demand. The reason for this popular acceptance is that since the porous materials having large pore volume ratios excel in ability to provide heat insulation and ability to abate noise besides the ability of absorption, they can be used in various fields covering building materials, audio products, and gardening articles. In producing a porous material by the HIPE method, therefore, it is stated in U.S. Pat. No. 5,334,621, for example, that it is particularly commendable to set the pore volume ratio at 97 wt. % (W/O=33.1). Generally, the practice of setting W/O in the range of 30/1-100/1 is frequently adopted. Specifically, an effort to produce a porous material according to the HIPE method entails consumption of a large amount of water for the formation of a water-in-oil type high internal phase emulsion. This fact implies that the production of the porous material by the HIPE method entails the necessity of dehydrating and drying the porous material at the end of the production because the material has contained water during the course of production and, therefore, ends up in generating waste water in a large quantity.
Moreover, the HIPE method generally uses a monovalent--trivalent inorganic salt of an alkali metal for the purpose of incorporating the salt capable of producing a salting out action into the water phase thereby allowing emulsification to proceed smoothly and, at the same time, enabling the produced water-in-oil type high internal phase emulsion to be improved in stability. The waste water obtained by dehydrating the porous cross-linked mass resulting from a polymerization reaction and a cross-linking reaction and the waste water remaining after the washing of the porous material, therefore, copiously contain salts and various kinds of impurities such as unaltered polymerization initiator and decomposed polymerization initiator and show changes in pH. In other words, since the HIPE method produces the porous material via a process for the preparation of a water-in-oil type high internal phase emulsion, the supply of water in a large quantity causes a serious problem and, at the same time, the disposal of a large quantity of waste water containing used salts poses an enormous problem. The waste water, when discarded in its unaltered state, exerts an extremely large load on environment, whereas the waste water, when so treated in advance as to eliminate completely the load on environment, calls for enormous expense and labor.
The porous material mentioned above can be used as sound insulating material and heat insulating material for absorbing sound and heat and as chemical impregnating substrate for impregnating perfume and detergent and further as absorbent for oil and organic solvent. When it is used as a sanitary material such as a disposable diaper or a sanitary article or as a cosmetic article or a medical supply which by nature is destined to contact directly the human body, the porous material in such a finished product is required to possess a surface adapted to minimize the stimulation exerted on the skin so as to prevent the user from suffering eruption of the skin and experiencing unpleasant feeling.
The surface of the porous material which is obtained by polymerizing a water-in-oil type emulsion produced by the conventional method, however, possibly manifests such a low pH as 2-3. The conceivable cause for this low pH value may be that the product of decomposition of the polymerization initiator included in the raw material and the hydrolyzate of the polymerizable monomer are suffered to survive as deposited on the produced porous material. Thus, the method which resides in rinsing with water the porous cross-linked polymer obtained by polymerizing a water-in-oil type emulsion thereby removing simulative substances including acidic substances has heretofore been adopted. This method, however, brings such unfavorable results as increasing the cost of production and exalting the load on environment because it requires to use the rinsing water in an amount several tens times, occasionally some hundreds times, it requires to perform the work of treating and discharging the waste water occurring in a large amount.
In contrast, simply decreasing the amount of water to be used is no easy thing. For, the proportion, wt. %, of the water phase in the water-in-oil type high internal phase emulsion cannot be decreased because the mechanical properties and the ability of absorption of the porous material are governed by the aforementioned ratio of W/O. Since the porous material is used as the core member for a sanitary material and as the material for treating waste oil and as the heat insulator and the sound insulator, it is preferred to possess minute pores in the texture thereof for the purpose of securing characteristic properties fully satisfactory for the purposes of such applications. When the proportion of the water phase is so low as mentioned above, the porous material comprising fine open cells is not obtained and the porous material produced at all suffers the mechanical properties to be deteriorated and the absorption characteristics to be degraded. In the circumstance, the desirability of developing a process which generates waste water in a decreased amount or generates no waste water, affects the environment tenderly, and promises a decrease in the cost of disposal of waste water has been desired.